The present invention relates to web supporting and drying apparatus. In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation, in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example.
One example of such a dryer can be found in allowed U.S. patent application Ser. No. 07/607,261, the disclosure of which is hereby incorporated by reference. That patent application discloses an air flotation dryer with a built-in afterburner, in which a plurality of air bars are positioned above and below the traveling web for the contactless drying of the coating on the web. In particular, the air bars are in air-receiving communication with an elaborate header system, and blow air towards the web so as to support and dry the web as it travels through the dryer enclosure.
Various attempts have been made in the prior art for decreasing the length and/or increasing the efficiency and line speed of such dryers. To that end, infrared radiation has been used either alone or in combination with air to dry the web. For example U.S. Pat. No. 4,936,025 discloses a method for drying a moving web by passing the web free of contact through various drying gaps. Thus, the web is passed through an infrared treatment gap in which infrared radiation is applied to the web from an infrared unit, and then is passed into an air-drying gap within which the web is dried by gas blowings from an airborne web dryer unit which simultaneously supports the web free of contact.
U.S. Pat. No. 4,756,091 discloses a hybrid gas-heated air and infrared radiation drying oven in which strips of infrared heaters are arranged with heated air inflow nozzles alongside thereof. The dryer is not a flotation dryer, and the air impingement nozzles and infrared elements are mounted on only one side of the web.
Conventional infrared dryers where infrared radiation is used alone do not scrub the boundary layer of the drying surface. This causes an excessive temperature rise with insufficient drying on the web surface due to the high humidity in the boundary layer. In addition, standard infrared loses 70% of its heat to surroundings by transmission, reflection, and radiation which does not fall on the surface to be dried. This results in poor efficiency and an uncomfortable and/or hazardous working environment. Moreover, installing infrared radiation means in conventional convection dryers typically increases the dryer length, and is often difficult or impossible in view of the limited existing space between air bars.
It is therefore desirable to have a combination infrared and air flotation dryer without substantially increasing the size of the dryer. In addition, it is desirable to be able to retrofit existing air flotation dryers with infrared burners in order to take advantage of the added drying capacity provided thereby, again without substantially increasing the dryer size.